Internal combustion engines on the basis of gasoline engines are generally operated with fuel from hydrocarbons, from fossil fuels based on refined crude oil. Ethanol produced from renewable resources (plants) or another kind of alcohol is increasingly being added in various mixing ratios to the fuel. In the USA and Europe a mixture of 75-85% ethanol and 15-25% gasoline is often distributed under the trade name E85. The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with mixtures up to E85. This is denoted as a “flex-fuel operation”. The operating parameters in the flex-fuel operation have to be adapted in each case to the existing fuel mixture for an efficient operation with only a small discharge of toxic emissions; while at the same time high engine performance is guaranteed. A stoichiometric fuel-air mixture ratio is, for example, present at 14.7 mass parts of air per part of gasoline; however, when using ethanol, a proportion of air of 9 mass parts must be set. Fuels with different percentages of ethanol with respect to gasoline have a different octane number. For example, high test gasoline has a ROZ (research octane number) of at least 95, while pure ethanol has a ROZ of at least 108. Mixtures of both types have ROZ numbers lying between the two values. This means that both types of fuel and the mixtures produced from them have a significantly different knock behavior. This must be taken into consideration during the operation of the internal combustion engine in order on the one hand to achieve a good degree of efficiency and on the other hand to avoid damage to the internal combustion engine. Small and/or slow changes in the alcohol content can be detected and taken into account by the engine management system of the internal combustion engine by means of a lambda probe and/or a knock sensor. Rapid changes with a significant deviation in the composition of the fuel mixture can however occur, for example, after filling the tank (fueling). If the internal combustion engine were operated with 100% gasoline and filled with E85 when the tank was close to empty, problems in starting and disturbances in the combustion can arise, which can also increase the harmful exhaust gas emissions. According to the state of the art, such rapid changes in the composition of the fuel can be detected using an ethanol sensor. This component, however, increases the cost of the internal combustion engine.
It is the task of the invention to provide a method, which allows for a reliable, cost effective detection of the composition of a fuel mixture from gasoline and ethanol or of another fuel mixture from fuels with different octane numbers.